Disclosed is an extrusion die assembly useful for continuously controlling the flow of molten polymer, thereby facilitating the process for extruding low molecular weight polymer melts.
This invention relates to the manufacture and processing of polymer in molten form, i.e. liquid or semi-liquid, where the polymer is extruded through a nozzle then is cut into discrete pieces for further processing. In conventional polymer melt cutting, the melt is distributed to a plurality of orifices drilled radially in a die plate. The plate is usually heated with a combination of band heaters and cartridge heaters to keep the polymer flowing and to avoid frictional drag on the polymer passing through the die plate and to prevent solidification prior to cutting. This device works well with high molecular weight polymers because, as individual holes start to plug due to local freezing of the polymer from exposure to the liquid sweeping the cut particles from the face of the die plate, increased pressure is generated upstream of the die plate which tends to clear frozen polymer from the plugging hole(s). In melt cutting low molecular weight polymers, the same freezing phenomenon occurs, but, because of the low melt viscosity, sufficient pressure to clear frozen material from the plugged hole(s) is not generated upstream of the die plate. Subsequently, additional holes begin to freeze off completely. This freezing process continues until a small number of holes remain open to polymer flow. This, of course, reduces the efficiency of the process and results in poor cut quality and inconsistency in particle size.
Various references have addressed the field of melt cutting polymer and other extruded matter of high viscosity. However, none of the below references has specifically addressed the problems of controlling the flow of lower viscosity materials during a melt cutting operation.
Schippers, U.S. Pat. No. 3,521,325 discloses an apparatus wherein a circular nozzle plate is surrounded on the outer circumferential surface by an induction coil within a housingxe2x80x94an alternating current is applied to the induction coils.
Nakamura, U.S. Pat. No. 4,421,470 describes a die having holes which are interspersed by heaters so the die does not cool down when cooling water is forcibly jetted against the die surface as molten resin is extruded.
Mattera, U.S. Pat. No. 5,215,763 discloses apparatus in which three band heaters are mounted around or within a die plate; sufficient heat is produced to maintain the polymer at optimum temperature and viscosity for extrusion.
In Irvin, U.S. Pat. No. 5,192,543 a heated die plate for making extruded pasta shapes can be modified in any practical manner to be used with six core heaters (100-400 watts) controlled by a thermostat. This patent also discloses that each profile may be heated individually by supplying heat to each hole, e.g., through resistance wires.
In Scotto, U.S. Pat. No. 3,775,035, thermoplastic material is extruded through a die having independently controllable heaters around the diexe2x80x94temperature is varied based on measurement of the thickness of the websxe2x80x94die has tubular orifice; tubular extrusion is expanded by air under pressure and a knife splits the tube longitudinally to form two sheetsxe2x80x94the heat applied is non-uniform, and is applied circumferentially around the die.
Bentivoglio, U.S. Pat. No. 4,830,595 describes die lip heaters that heat just the surface of an extruded plastic film (tubular shape). A ring of heat conductive material is positioned at the lip and is heated by coils of electrical resistance wires.
Vermeerbergen, U.S. Pat. No. 3,819,777 describes temperature control of a multiple-head extruder based on pressure readings. Electrical resistance heaters, extrusion heads, and cooling means are also disclosed.
Disclosed is an improved extrusion die assembly for extruding polymer melts, comprising:
(a) a plurality of extrusion barrels, each barrel comprising an inlet end, an outlet end, and an interior cavity, the interior cavity containing polymer melt which is conveyed from the inlet end to the outlet end;
(b) a plurality of heaters, each heater having a hollow, cylindrical shaped interior, and each heater containing a corresponding extrusion barrel within the interior, such that the corresponding extrusion barrel is heated to a predetermined temperature;
(c) a die plate, having an outer die face, and a plurality of die plate cavities, each of the die plate cavities forming a housing for supporting one of the plurality of extrusion barrels and heaters, each extrusion barrel and corresponding heater disposed within a corresponding die plate cavity, and each of the die plate cavities having an air gap between the heater and the die plate cavity inner surface;
(d) a plurality of devices for the direct or indirect measurement of polymer melt flow, each flow measuring device disposed in a corresponding extrusion barrel outlet in contact with the polymer melt at a location immediately interior of the extrusion barrel outlet; and
(e) at least one controller taking signals from said flow measuring devices and directing one or more of the plurality of heaters to supply heat as needed to the extrusion barrel outlets to maintain preselected temperatures.
Various other embodiments are taught herein.